CODEWITHSUNDEEP
sslopenssl
user1503944
user1503944

Reputation: 397

TLS Extended Master Secret- calculate session hash

I'm have tool which manually dissect ssl/http traffic using openssl library. It's work fine in most cases, but failed wheen client/server usese Extended Master Secret extension.

Fail occured on last stage of handshake, where veryfying of test encripted data performed .

As I understand for proper work of this extension I need properly fill handshake buffer with specific TLS fields (client hello, server hello, key exchange) and then master key should be generated baasing on hashing of handshake buffer.

Unfortunatelly it's not work for me.

So my question-which exactly tls packeds should be hashed for generating proper master key ?

Will be gratefull for any comments. Thanks.

PS: I downloaded experemental version of openssl from github, where this functionality implemented.

openssl s_server/s_client uses corespond extension, Ican see it in Wireshark, and able to decode traffic with server pem file.

Upvotes: 1

Views: 2087

Answers (1)

Stiju
Stiju

Reputation: 26

You need to do the following to generate the session hash

1) Append all the Handshake messages, other than encrypted handshakes in order of arrival, please don't include the record header to the handshakes. only messages and message headers

2) Hash them based , simple to say, Hash algorithm varies based on Cipher Suite and Protocal Version TLS1 and TLS1.1 its mixture of SHA1 and MD5 (16 bit each) TLS1.2 its mostly SHA256 or SHA384 based on cipher suite.

Upvotes: 1

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